This invention is particularly applicable for high frequency inverter used for DC welding, wherein a rectified D.C. power supply is ultimately switched to produce alternate polarity output pulses in the secondary of a transformer means, wherein the output pulses are rectified and directed across the electrode element and workpiece of the welding station for the purposes of performing a welding operation. The present invention is described with reference to this use of a specific inverter; however, the invention has broader applications and may be used with various high frequency inverters of the type creating a series of current pulses which create current directed across the electrode and workpiece of a welding station for the purposes of performing a welding operation. In accordance with the preferred application of the invention, the switching operation is accomplished by two separately and alternately operated switch means, such as FET's, wherein one switch means is activated to magnetize the core of a transformer to produce an output current pulse in the secondary of the transformer and the other switching means is employed for creating a separate opposite polarity output current pulse at the secondary of the transformer. By alternating switching the operation of the two switch means, high frequency alternating current is magnetically coupled into two secondary windings of the transformer. This high frequency current in the form of pulses is directed by rectifying means, such as high speed diodes, through electrical filtering means, such as an inductor or choke, to the output terminals of the D.C. welder.
The secondary stage of the output transformer for an inverter, of the type to which the present invention is particularly directed, has two secondary windings in which are created current pulses that are separately rectified. When the output of the transformer is driven by a first primary winding, there is a pulse of current created in a first electrical polarity in the power supply. A pulse of the opposite electrical polarity is then created in the second secondary winding at the output of the transformer of the power supply by a pulse in a second primary winding. These opposite polarity current pulses are directed through the high speed diodes to the welding station. High frequency electrical current pulses of proper polarity are thus directed toward the inductor or choke of the welder, through the inductor or choke to the output terminals of the D.C. welder.
The first and second switch means for creating the first and second current pulses are controlled by electrical trigger or gate pulses of a selected pulse time or pulse width and repetition rate. To control the magnitude of the welder output current or voltage, the width, or spacing, of the trigger pulses is varied to vary the output current across the electrode and workpiece of the welding station or installation. As more current is required for the welding operation, the width of the pulses for energizing the primaries of the transformer is increased in proportion to the desired increase in the output current. The current is normally controlled by a feedback circuit for the inverter by use of a voltage controlled pulse width modulator circuit operated at a selected frequency, such as 20 kHz. These modulators are commonly used in electrical switch mode inverter power supplies and are available in the form of standard integrated circuit packages familiar to those skilled in the art. The selected frequency of operation affects the response time of the welder to the feedback current signal or senses voltage signal and, thus, affects the ability of the welder to respond to small changes in the welding arc. A high frequency, above approximately 10 kHz, is necessary to provide substantial improvement in performance of the inverter. The frequency of operation also affects the audibility of the arc and the welding circuitry. To minimize objectionable noise and improve operator appeal a frequency above approximately 20 kHz is normally chosen.
In accordance with the invention of Bilczo U.S. Pat. No. 4,897,522, the high frequency inverter to which the present invention is particularly directed, is modified by the inclusion of a booster winding on the output of the power transformer for providing additional voltage to extend the operating current range of the inverter. The input or primary windings for the power transformer tend to saturate the core of the tightly coupled transformer design where pulses in one direction have a different time duration than pulses in the other electrical direction. By having different current flows through the transformer, the D.C. component is created in the core causing a large D.C. current to flow. This current flow results in a large ampere-turn product and forces the D.C. flux in the transformer to increase for saturation of the transformer core. Consequently, the novel design in Bilczo, although highly advantageous over known structures, still presented a problem in obtaining appropriate utilization of the ferrite core material of the transformer. Further, the prior power supply was somewhat unstable at lower welding currents and presented some difficulties with respect to starting of the welding operation.